124 Estimates of the daily rate of endochondral os- sification, 1.e., linear growth of the tibias, were made by dividing the total distance (in microns) that the trabeculae labeled at time 0 had been displaced from the epiphyseal dise by the number (3) of days. RESULTS The weight gain, linear bone growth characteristics, and the appearance of the cartilage plates of the rats maintained on the various diets were comparable to those described previously.“®) For the description of the pattern of labeling in cartilage, we have selected the terminology of Balogh and Kunin.‘? Balogh and Kunin“) Equivalent to Dodd’s“®classification Upper layer Zone 1—Narrow ‘reserve vell’’ zone adjacent to the bone and marrow spaces of the epiphysis. Middle layer Zone 2—Zone of cell proliferation composed of wedge-shaped or discoidal ¢ells. Zone 3—Zone of cell enlargement. Lower layer Zone 4—Zone of cell hypertrophy. The data relative to appositional growth rates of bone and OAT ratios have been tabulated in Table TABLE 58. AvERAGE Griin Counts (#H-PROLINE) iN OSTEOBLASTS DURING DEVELOPMENT AND HEALING OF Rickets IN Rat Tupras Post- injection time of Group : Fen sacri- O'F8™ Shaft end- a Epiphy . osteum [ett PORE fice) Controls: ad libitum-fed th 13.1 10,9 10.9 3d 5.4 6.5 5.1 Controls: pair-fed 4h 3d 13.0 6.1 i1.1 6.5 13.8 5.9 Rachitic (3 wk) 4h 3d 12.4 5.5 12,5 5.0 15.0 6.2 Rachitic + phosphorus (1 4h 12,2 15.4 11.4 3d 5.2 5.2 5,4 7.6 10.3 10.1 8.1 4.2 10.1 5.8 10.5 5.1 4h 8.5 11.3 12.2 3d 5.4 5.7 6.2 wk) Rachitic ++ vitamin D th 3d | 4h 3d | | 4.96 6.5 6.2 Control Animals Fed Commercial Laboratory Che At 4 hr after injection, silver grains were localize predominantly over the cells (nucleus and cyte plasm) and extracellular matrix in the middle lay: of the cartilage (Figure 96). Perhaps only 2% of tl cells in the upper layer were labeled. The matri: nuclei and cytoplasm of all the cells in the lowe zone were labeled as well, but the intensity of th autoradiographs was much less than that noted i the middle layer. The cores of cartilage in the tra beculae of the primary spongiosa were not labeled a this early time. Radioproline tagged the undifferen tiated and differentiated cells in the metaphysis, an: large concentrations of grains were found between th osteoblasts and bone trabecular surfaces. The osteo blasts were the most intensely labeled, and this gen eral pattern was found on all periosteal and endostea surfaces which were undergoing appositional bon: growth at the time of injection. Newly buried osteocytes in the metaphysis and shaft were also market by radioproline, and the tracer was located at the edges of their lacunae. Only the cytoplasm of osteoclasts was labeled. Significant labeling of the extracellular matrix of the cartilage plate was evident 3 days after tracer injection, but the tracer was concentrated in the middle and lower layers only. The intensity of the autoradiographs over the middle layer was less than at 4 hr, and somewhat increased over the lower layer. The remnants of the cartilage which had initially been labeled and had subsequently undergone endo- chondral ossification now projected from the plate into the metaphysis in trabeculae of the primary spongiosa, and lamellae of primary trabecular bone in this area were more diffusely labeled than the cores of cartilage. Distally, the trabeculae were very heavily labeled. This region quite obviously marked the juxtaepiphyseal zone labled by radioproline at 4 hr which had been displaced during the 3-day postinjection period by continued endochondral ossification and linear bone growth (200-220 »/day). In this time, the trabceulae grew thicker by appositional bone, growth and this process served to bury the heavily» labeled Jamellae under more diffusely labeled col- , lagen matrix. The cytoplasm of the osteoblasts in ° Control: Rachitic + phosphorus (3 wk) + vitamin D | | () Average of 3 rats per time interval. oned this zone showed some 5-6 grains. It was impossible | to measure the appositional rate of bone growth on a Re ter S Bka Rachitie + phosphorus + vitamin D (1 wk) a. fies (1 wk) 57. Osteoblast grain counts have been included Table 58. these trabeculae owing to their irregular contours. In the animals fed Purina Chowad libitum, the , daily apposition rate of Iamellar bone measured on the periosteum, shaft endosteum, and transverse epi-